Formation of Three or Four Bonds

Synthetic methods for the preparation of six-membered heterocyclic systems which proceed via the formation of three or four bonds are virtually restricted in application to the monocyclic heterocycles and have been most widely applied to pyridine and pyrimidine derivatives. In principle, reactions which proceed with the formation of three ring bonds can be sub-classified into three groups, namely, those involving [4 + 1 + 1] atom fragments, [3 + 2 + 1] atom fragments and [2 + 2 + 2] atom fragments. 

Ring Syntheses which Involve Formation of Three or Four Bonds... 

Formation of three or four bonds Synthesis by Transformation of Other Heterocycles... 

Asymmetric halolactonization is a much used procedure for the stereoselective formation of C — Br and C —I bonds (see also Sections D.4.6. and D.7.2.). This intramolecular reaction is used to transfer chiral information in the molecule over three or four bonds, sometimes with high stereoselectivity. Formation of a halonium complex with the olefin is the first step of the reaction followed by intramolecular lactonization. Induction of asymmetry is achieved in different ways. 

In common with other first-row elements, nitrogen has only four orbitals available for bond formation, and a maximum of four 2c-2e bonds may be formed. However, since formation of three electron-pair bonds completes the octet, N( R)3, and the nitrogen atom then possesses a lone pair of electrons, four 2c-2e bonds can only be formed either (a) by coordination, as in donor-acceptor complexes, e.g., F3B—N(CH3)3, or in amine oxides, e.g. (CH3)3N—6, or (b) by loss of an electron, as in ammonium ions NH, NR 4. This loss of an electron gives a valence-state configuration for nitrogen (as N+) with four unpaired electrons in sp3 hybrid orbitals analogous to that of neutral carbon, while, as noted above, gain of an electron (as in... 

Initially these hyper-valent compounds were thought to result from the promotion of electrons from xenrai valence orbitals into higher-lying d orbitals and subsequent formation of two-centre two-electron bonds with O or F, i.e. they exceed the octet rule. Contemporary interpretations of the bmiding in these compounds favour the formation of three-centre four-electron bonds. This interpretatiOTi results in the ionic canonical forms illustrated in Fig. 12, which when in resonance reproduce the observed symmetric geometries. In these molecules, x represents the number of three-centre four-electron bonds in the molecule [101]. 

Syntheses of alkenes with three or four bulky substituents cannot be achieved with an ylide or by a direct coupling reaction. Sterical hindrance of substituents presumably does not allow the direct contact of polar or radical carbon synthons in the transition state. A generally applicable principle formulated by A. Eschenmoser indicates a possible solution to this problem //an intermolecular reaction is complex or slow, it is advisable to change the educt in such a way. that the critical bond formation can occur intramolecularly (A. Eschenmoser, 1970). 

The Tetrahedral Carbon Atom.—We have thus derived the result that an atom in which only s and p eigenfunctions contribute to bond formation and in which the quantization in polar coordinates is broken can form one, two, three, or four equivalent bonds, which are directed toward the corners of a regular tetrahedron (Fig. 4). This calculation provides the quantum mechanical justification of the chemist s tetrahedral carbon atom, present in diamond and all aliphatic carbon compounds, and for the tetrahedral quadrivalent nitrogen atom, the tetrahedral phosphorus atom, as in phosphonium compounds, the tetrahedral boron atom in B2H6 (involving single-electron bonds), and many other such atoms. 

Despite the study of cycloadditions to alkylidenecyclopropanes being far from fully exploited, the results accumulated in the literature are enough for a comprehensive review which will present the state of the art. The present review will deal with all the processes of cycloadditions, without discriminating between the nature, concerted or stepwise, of the processes. It will cover those reactions which involve exclusively the exocyclic double bond of methylenecye-lopropane and alkylidenecyclopropanes in the formation of three-, four-, five-and six-membered rings. 

A wide variety of cyclic S-N compounds containing two or three coordinate sulfur is known (i ), The binary S-N rings vary in size from four (S2N2) to ten (SsNs" ") atoms and examples of anions (SsNs") and cations (S3N2 , S N3 , S N ) are known in addition to neutral molecules (2). The excess of electrons in these planar, electron-rich heterocycles is often accommodated in low lying tt orbitals but, in some cases (e.g. S N, S Ns"), the number of tt electrons is reduced by formation of transannular S-S bonds to give cages ( , ). 

---